The dual fluid heat (e.g. Cheng cycle) engine has been described in U.S. Pat. Nos. 3,978,661, 4,128,994 and 4,248,039. Parameter optimization and control paths for the dual fluid heat engine are described in U.S. Pat. Nos. 4,297,841, 4,417,438 and 4,393,649. This invention relates to an improved design for, and improved procedures for operating, dual fluid heat engines utilizing the Cheng cycle as well as conventional systems, and other dual fluid systems.